This invention relates to media for reading and recording magnetic data, and more particularly to a thin film magnetic medium for longitudinal recording.
As media for recording and reading magnetic data, magnetic thin films have gained acceptance and often are preferred over oxide or other particulate media. This is due to favorable magnetic properties in metallic thin films, in particular higher remanence magnetization and coercivity. Higher magnetization levels yield higher amplitude read-back signals for a given film thickness. Higher coercivity increases resistance to demagnetization, for improved long term data storage. Large coercivities also give rise to shorter magnetic transition lengths between data bits, thus increasing maximum storage densities for a given remanence magnetization and film thickness.
A disadvantage associated with magnetic thin films for longitudinal recording is their relatively high levels of noise, resulting in signal-to-noise ratios less than those of particulate media in spite of the enhanced signal amplitudes. Further, the noise power observed for longitudinally recorded thin films is strongly dependent upon the density of written transitions. A primary cause of noise in thin film media is believed to be intergranular coupling in dense, generally continuous films, where exchange coupling and magnetostatic interactions among neighboring grains are relatively strong. This causes distortions of the ideal linear transition between recorded data bits to form irregular or zigzag transitions. Such transitions have been observed by Lorentz microscopy, electron holography, and spin polarized scanning electron microscopy.
In contrast to magnetic thin films, oxide media and other particulate media comprise magnetic particles dispersed in a non-magnetic binding material. With individual particles separated from one another, interactions are greatly reduced. Non-uniformity in particle size, clustering of particles, and surface roughness are the principal sources of noise.
Vertical cobalt chromium magnetic media, in contrast to longitudinally recorded media, exhibit relatively low noise. However, vertical recording requires higher write currents and is not practical for most rigid disk applications.
Those skilled in the art are aware of efforts to improve the noise characteristics of thin film media with data recorded in the longitudinal mode. For example, J. I. Lee, in Paper AB-11 presented at the 1987 Intermag Conference, disclosed that cobalt chromium films (23 percent Cr by weight) deposited by RF-sputtering, ranged from vertical to isotropic-to-longitudinal orientations, all with the CoCr films being approximately 8700 angstroms thick. The isotropic-to-longitudinal medium, recorded in a longitudinal mode, was found to exhibit low noise behavior similar to the vertically recorded medium. However, it had a coercivity of 317 oersteds, much less than desired values of at least 600 oersteds.
Christner et al. ("Low-Noise Metal Medium for High-Density Longitudinal Recording", Journal of Applied Physics, Vol. 63, No. 8, Apr. 15, 1988), disclosed that CoCr based ternary thin films deposited on a chromium underlayer exhibited low noise and satisfactory coercivity (1100 oersteds). The film included about 15 percent Cr and up to 10 percent of a third transition metal element. The grain structure of the film replicated that of the chromium underlayer. The coercivity was found to increase sharply with thickness of the chromium underlayer, up to a thickness of about two thousand angstroms. Increasing underlayer thickness also was found to improve the magnetic orientation from isotropic to a preferred longitudinal orientation. It was suggested that the low media noise of the cobalt-chromium based ternary alloy was the result of relatively weak interactions among grains or other magnetic switching sub-units, due to the separation of the cobalt rich regions from one another by non-magnetic regions rich in chromium. This type of separation has been observed for vertical cobalt-chromium media. For such a system, the very strong but short range exchange interactions would be quenched, and the longer range magnetostatic interactions would be decreased.
U.S. Pat. No. 4,786,564 (Chen et al.) discloses a method for manufacturing a magnetic disk with a nickel-phosphorus alloy layer on an aluminum substrate, a second nickel-phosphorus layer sputtered upon the first, and a magnetic alloy sputtered upon the second nickel phosphorus layer. The magnetic alloy is sputtered immediately after sputtering of a previous layer, and is controlled as to the nucleation and growth of the crystalline structure to prevent non-uniformities in the substrate surface from affecting the magnetic characteristics of the magnetic media. The sputtered nickel-phosphorus layer is nearly amorphous and sufficiently thick (e.g. from 5 to 200 nanometers) to prevent non-uniformities in the underlying substrate surface from being transmitted to the magnetic layer.
Further efforts have been directed to improving performance of a metallic recording layer through controlling an underlayer of chromium, although not directed to media noise. For example, Fischer ("Magnetic Properties and Longitudinal Recording Performance of Corrosion-Resistant Alloy Films", Fischer et al., IEEE Transactions on Magnetics, Vol. Mag-22, No. 5, September 1986) considers CoCr thin films in comparison to CoCrTa films, particularly as to corrosion resistance and coercivity. Films were sputter deposited in a DC magnetron system, using an argon pressure of ten millitorr. The films were deposited on a chromium underlayer, and the nature of the underlayer, in particular its thickness, was found to influence coercive force. In particular, coercivity was found to increase to a maximum value at a chromium underlayer thickness of five thousand angstroms, with the CoCrTa films significantly higher in coercive force.
Allen ("A Comparison of the Magnetic and Recording Properties of Sputtering Ternary) Alloys for High Density Application", Allen et al., IEEE Transactions on Magnetics, Vol. Mag-23, No. 1, January 1987) discloses a number of ternary alloys including CoCrTa, CoNiCr, CoCrW and CoNiW, with chromium atomic percentages ranging from twelve to nineteen percent. These alloys were sputter deposited upon a chromium underlayer using a ten millitorr argon pressure. Coercive force was found enhanced by the third element, and further dependent upon the thickness of the sputtered chromium underlayer.
While the above activity has led to increased understanding of the nature of metallic thin film media utilized for longitudinal recording, there remains the need for a metallic thin film affording the high amplitude read signal and high coercivity characteristics of present continuous grain structures, but with the further advantage of the relatively low noise level of particulate media.
Therefore, it is an object of the present invention to provide a longitudinal metallic thin film exhibiting low noise behavior largely independent of the density of magnetic flux changes.
Another object of the invention is to provide longitudinal thin film media having a coercivity of at least six hundred oersteds and a high remanence and magnetization for a relatively high amplitude read signal, in combination with noise levels characteristic of particulate media.
Yet another object of the invention is to provide a metallic thin film suited for longitudinal recording, having a desirably low percentage of the non-magnetic components of the alloy, yet with individual grains sufficiently isolated from adjacent grains to yield low noise behavior.